This invention relates to a linear aromatic polyester comprising bisphenol and dicarboxylic acid monomer residues, the carboxylate end groups of which are structurally modified to provide a product of enhanced hydrolytic stability. More particularly, the invention relates to such polyesters which contain carboxylic acid ester terminal groups which are derived from certain monofunctional fluorine-substituted aliphatic alcohols, i.e. which contain fluoro-alkyl oxy carbonyl ester terminal substituents.
Linear aromatic polyesters prepared from aromatic dicarboxylic acids and bisphenols are well known for their suitability for molding, extrusion, casting, and film-forming applications. For example, U.S. Pat. No. 3,216,970 to Conix discloses the preparation of linear aromatic polyesters comprising monomer residues of isophthalic acid, terephthalic acid, and a bisphenolic compound by the poly-esterification reaction of a bisphenol (inclusive of a biphenol) with the acyl halides of terephthalic and isophthalic acids. Such high molecular weight compositions are known to be useful in the preparation of various films and fibers. Further, these compositions, when molded into useful articles using conventional techniques, provide properties superior to articles molded from other linear polyester compositions. For instance, aromatic polyesters are known to have a variety of useful properties, such as good tensile, impact, and bending strengths, high thermal deformation and thermal decomposition temperatures, resistance to UV irradiation and good electrical properties.
Aromatic polyesters which are particularly well suited for molding applications may also be prepared by reacting an organic diacid halide with a difunctional aliphatic reactive modifier, such as a glycol, and subsequently reacting this product with a bisphenol compound. The resulting polyesters have reduced melt viscosities and melting points which facilitates molding at temperatures within the operable limits of conventional molding apparatus (i.e. temperatures and pressures of less than about 300.degree. C. and about 20,000 psi respectively). This type of glycol-modified polyester is more fully disclosed in U.S. Pat. No. 3,471,441, to Hindersinn et al.
In order to form a successful molding resin on a commercial scale, a polymer should be capable of being molded conveniently without significant degradation in physical properties. In this respect, although the aforementioned aromatic polyesters generally display excellent physical and chemical properties, a persistent and troublesome problem has been their sensitivity to hydrolytic degradation at elevated temperatures. This sensitivity to the combined effects of heat and moisture is also exhibited in commercially available polycarbonate resins as evidenced by the desirability of reducing the water content of the resin to less than about 0.05% prior to molding. However, the aromatic polyester resins often display a more pronounced tendency to rapidly degrade and embrittle than do polycarbonate resins. This is demonstrated by the loss of tensile strength which can occur when an aromatic polyester resin is molded and subsequently immersed in boiling water.
It has been proposed (according to T. Ueno et al., Japanese Patent No. Sho 53-8696, published Jan. 26, 1978) to enhance the hydrolytic stability of linear aromatic polyesters comprising bisphenol and dicarboxylic monomer residues by carrying out the preparation of the polyester from the bisphenol and dicarboxylic acid diacyl halide reactants in the presence of a monohydric phenolic compound such as p-tert. butyl phenol. The monofunctional phenolic compound reacts with terminal carbonyl halide substituents in the polyester to form a carboxylate ester end group derived from the monofunctional phenolic reactant. However, as is illustrated by data in the Table below, modification of the polyester to introduce terminal carboxylate ester groups derived from monofunctional phenols (such as p-tert. butyl phenol) does not enhance the hydrolytic stability to make the products attractive commercially.
In the copending U.S. application of J. C. Rosenfeld Ser. No. 045,464 filed of even date with this application, there are disclosed linear aromatic polyesters comprising bisphenol and dicarboxylic monomer residues which contain carboxylate end groups derived from monofunctional hydrocarbon alcohols of 8 to 45 carbon atoms and which possess improved hydrolytic stability. Another copending U.S. Application Ser. No. 045,461 of G. Salee and J. C. Rosenfeld, filed of even date with the present application discloses an improved process of preparing such polyesters by transesterification polymerization.
It is the principal object of the present invention to provide linear aromatic polyesters of enhanced hydrolytic stability with substantial retention of the other beneficial physical and chemical properties of the polyester.